Bson Serialization/Deserialization

Bson.cs

void Main()
{
    var bson = new MongoDB.Bson.BsonDocument(new Dictionary<string, object>
    {
        ["ChildishField"] = "childish",
        ["AnotherChildishField"] = "more childish"
    });
    var serialized = bson.ToString().Dump();

    MongoDB.Bson.Serialization.BsonSerializer.Deserialize<Child1>(serialized).Dump();

}
[BsonIgnoreExtraElements]
[BsonKnownTypes(typeof(Child1))]
public abstract class MyAbstract
{
    public string DontCare;
}

[BsonIgnoreExtraElements] // comment this out and it throws as expected
public sealed class Child1 : MyAbstract
{
    public string ChildishField;
}